Bicyclists interested in self-preservation learn quickly that it's not a good idea to pedal against traffic, and safety experts discourage the practice, saying wrong-way riding is a leading cause of injury accidents.

But starting Friday on Polk Street, northbound bike riders will be encouraged to pedal in the opposite direction of one-way traffic - in the safety of a new bright green, protected, designed and landscaped "contra-flow" bike lane.

City crews have installed the against-the-grain lane on the short, one-way stretch of Polk between Grove and Market streets. The northbound bike riders are separated from traffic by a 6-foot-wide raised island that's planted with succulents. It's not the city's first contra-flow bike lane - there's one on Lyell Street near Alemany Boulevard - but it's the first that's been laid out by engineers as a landscaped and physically separated part of the street.

"What's different is the incredibly beautiful design and that it's protected," said Kristin Smith, a spokeswoman for the San Francisco Bicycle Coalition. "Even though it's just a couple of blocks, it shows what a radical change a small improvement can make."

While the wrong-way ride ends at Grove, where Polk becomes a two-way street, the lime-green painted lane continues past City Hall to McAllister Street. The existing southbound bike lane on Polk Street has also been widened and given the green treatment through the area, including where it passes City Hall.

The roadway has also been repaved from McAllister to Market and three sets of bike traffic signals have been installed at Market, Hayes and Grove streets along with a bike waiting area - green, of course - for those turning from eastbound Market to northbound Polk.

City officials plan to unveil the new contra-flow lane, and accompanying paint jobs and miscellaneous improvements, at an 11 a.m. ribbon-cutting at Market and Polk streets.

- Michael Cabanatuan

Back in action: Debbie Raphael, who spent more than a decade overseeing toxic regulations at the city's Department of the Environment before being hired to help spearhead statewide efforts at reducing hazardous chemicals, will return to San Francisco to lead the agency, Mayor Ed Leeannounced Thursday.

Raphael, who is well respected in the environmental field, will fill a position that has been vacant since January, when Department of the Environment director Melanie Nutter resigned unexpectedly.

Gov. Jerry Brown appointed Raphael to lead the state Department of Toxic Substances Control in May 2011, at a time when the state agency was struggling to implement regulations intended to ensure consumer products contain chemicals that are safe for human use and the environment. After five years of fits and starts, that law took effect last fall.

Lee said Raphael was chosen after an "extensive national search."

"Deborah Raphael is a proven leader who shares my commitment to extend San Francisco's environmental leadership in a way that is inclusive of our diverse communities and builds on our innovative economy," he said in a written statement.

Raphael has a bachelor's degree in biology and plant ecology from UC Berkeley and a master's degree in physiological plant ecology from UCLA. She previously worked as a life sciences exhibit designer at San Francisco's Exploratorium, an environmental program manager for the city of Santa Monica, and director of toxics reduction and green building for San Francisco.